In a liquid crystal display (LCD), the optical anisotropy and polarization characteristic of the liquid crystal molecule can be varied through controlling the orientation of the liquid crystal molecule, which in turn varies with the external electric field applied thereto, thus the refraction of light and display of images can be realized. Among various LCDs, the active matrix LCD has attracted much research and development and been widely used in consumer electronics and computers due to their high resolution and superiority in animation display. The active matrix LCD comprises thin film transistors (TFTs) and pixel electrodes arranged in matrix.
The LCD typically comprises an upper substrate, a lower substrate, and liquid crystal layer interposed therebetween. The upper substrate is called a color filter substrate, typically including a common electrode and a color filter. The lower substrate is called an array substrate, typically including TFTs and pixel electrodes. The color filter can be formed on the color filter substrate with several photolithography processes, and the TFTs and pixel electrodes arranged in matrix can be formed on the array substrate in 4-6 photolithography processes, each of which is carried out with repeated thin film deposition, exposure, etching, lifting off and the like. One mask is used in each circle of photolithography process
During manufacturing of TFT LCD, there always exists a need of reducing mask number and accordingly photolithography process number to reduce manufacture cost and improve equipment productivity. The manufacturing technology for TFT LCD array substrate has undergone the map from the seven mask (7Mask) technology to the current five mask (5Mask) and four mask (4Mask) technology used in mass production, and also three mask (3Mask) technology has been developed.
As mask number decreases, the structure of TFT is evolving continuously. The structure has evolved from co-planar type to normal staggered type, and further to back channel stop type and the back channel etching type, and has also evolved from top gate structure to current bottom gate structure. Removal of some device elements during the evolution of TFT directly results in decrease of the photolithography process number and mask number. For example, the bottom gate type TFT needs no light shielding layer that is used in the top gate type TFT, thus reducing one mask. Also back channel etching type TFT, as compared with the back channel stop type TFT, needs no the etch blocking layer, thus reducing another mask.
After improvement on TFT structure, methods for reducing mask number in the industry shift to the photolithography process itself, i.e., defining different patterns of two layers of thin films with one mask. As well known, the necessary elements of a LCD pixel unit comprise a gate electrode, a gate insulating film, an active film, an ohmic contact film and a source/drain electrode, a transparent pixel electrode, and a TFT passivation protection film. In the 5Mask process in mass production, the gate electrode, the gate insulating film and the active film as well as the ohmic contact film, the source/drain electrode, the passivation protection film, and the pixel electrode are separately formed in five photolithography processes with respective mask. However, for the 4Mask process, the gate insulating film, the active film, the ohmic contact film, and the source/drain electrode are formed with one mask in combination. This mask differs from any mask in the 5Mask process and is a so-called gray tone mask with narrow slits and bars. This mask forms stepwise photoresist with different thicknesses in different regions.
For the conventional mask, transparent and opaque portions are formed thereon and patterned the same as desired device pattern. The opaque portions are generally made of a metal film (e.g., Cr), while the transparent portions are void of any metal film. On the contrary, the gray tone mask additionally has partially transparent regions, e.g., slits with given width and spacing and arranged in order in given regions of the mask. The diffraction among the incident light changes transmitting ratio, so the photoresist corresponding to the partially transparent regions of the mask is subject to exposure different from that corresponding to the transparent regions, and the so-called photoresist partially exposed (gray tone) region is formed. Compared with the photoresist completely unexposed (full tone) region, the photoresist in the gray tone region is subject to partial exposure and is thinner than that in the full tone region.
The 4Mask technology using a gray tone mask will be explained below with reference to the drawings.
FIGS. 1a and 1b are diagrams illustrating an array substrate 100′ of typical back channel etching bottom-gate type TFT. The array substrate comprises a plurality of gate lines 1 and gate electrodes 2; a plurality of data lines 5 and source and drain electrodes 6 and 7; and pixel electrodes 10. A part of the gate electrode (gate protrusion part 11 overlapping with the pixel) and the pixel electrode 10 together constitute a storage capacitor. The TFT is manufactured with a 4Mask process. FIGS. 2a-4b show the top view of TFT substrate and cross-sectional view across the TFT at each intermediate stage of the process. The conventional manufacturing process comprises the following steps.
A gate metal film is formed on a transparent substrate, and a gate pattern, which includes the gate line 1 and gate electrode 2 as well as the gate protrusion part 11 for constituting the storage capacitor, is formed with the first mask by photolithography and etching process, as shown in FIGS. 2a and 2b. 
A gate insulating film 3, a semiconductor film 4 (e.g. an intrinsic semiconductor film), an ohmic contact film (not shown and e.g., a doped semiconductor film) and a source/drain metal film 15 are formed in order. Stepwise photoresist pattern as shown in FIG. 3b is defined with a second mask, a gray tone mask, wherein the photoresist 13 (partially retained in gray tone region) over the TFT channel is thinner than the photoresist 14 (completely retained in full tone photoresist) over the source/drain electrode and data line. The source electrode 6, the drain electrode 7, and the data line 5 are formed after etching, as shown in FIG. 3c. After the photoresist is completely removed in the gray tone region, the source/drain metal film 15 and the doped semiconductor film are further etched to form TFT conductive channel 12, as shown in FIGS. 3d and 3e. 
On the array substrate a second insulating dielectric film, i.e., the passivation protection film 8 is formed, and a third mask is used to define the protection film, as shown in FIGS. 4a and 4b, i.e., via hole 9 and connection pad (not shown) for the wires of the gate and source electrodes.
A transparent conductive film is formed on the passivation protection film 8, and a pixel electrode 10 is formed with a fourth mask, thereby achieving the TFT device as shown in FIGS. 1a and 1b. 
Based on the 4Mask technology, a 3Mask technology has been developed, in which the protection film and the transparent conductive film in the above process are completed in combination with a single mask. This modification is illustrated in FIGS. 5a-5f, in which the transparent pixel electrode pattern is defined by the photoresist lifting-off process used in the semiconductor integrated circuit. First, as shown in FIG. 5b, the photoresist 17 in the photoresist partially retained region (corresponding to the pixel electrodes) and photoresist 18 in the photoresist completely retained region are formed with a gray tone mask, and the photoresist-free via holes 16 are in the photoresist-free region. The passivation protection film via holes 9 are formed by etching through the photoresist-free via holes 16 (FIG. 5c); the photoresist 17 in the photoresist partially retained region is removed (FIG. 5d); a layer of transparent conductive film is formed in all the regions (FIG. 5e); finally the retained photoresist and the transparent conductive film deposited thereon are lift off, and the pixel electrode 10 and the conductive film 19 in the via holes are retained. The TFT structure manufactured by the lifting-off technology is shown in FIG. 5f. 
Compared with the conventional 4Mask technology, the 3Mask technology simplifies the manufacturing processes and improves the utilization ratio of the equipment, but it still suffers from the drawbacks such as complicated manufacturing process, low productivity, and low utilization ratio of the equipment.